Structure type II

The yellow regions in Figure 4.3 - 1 describe the crystallization fields of layered compounds with the composition Ln2(H2L), which correspond to structure type

II (Figure 4.3 - 2). Their crystallization was achieved by an increase of the metal- to-ligand ratio. In the La3+ containing system, four equivalents of NaOH facilitate the crystallization process (Figure 4.3 - 1a). With NaOH, La2(H2L) crystallized

between 75 °C and 100 °C at metal-to-ligand ratios between 2 :1 and 6:1. At 150 °C it could be observed at metal-to-ligand ratios of 4:1 and 6:1. Without NaOH, more drastic conditions were necessary. Then the formation of La2(H2L)

required at 75 °C a metal-to-ligand ratio of 6:1. Between 100 °C and 150 °C an excess metal-to-ligand ratio between 4:1 and 6:1 had to be provided. In the Nd3+ containing system four equivalents of NaOH are less favorable (Figure 4.3 - 1b). Without NaOH, Nd2(H2L) is formed at 75 °C at metal-to-ligand ratios between

4:1and 6:1. From 100°C to 150 °C it could be obtained at metal-to-ligand ratios between 2:1 and 6:1. In the presence of NaOH, Nd2(H2L) crystallized only at

75°C at a metal-to-ligand ratio of 2:1 and at 100 °C at a metal-to-ligand ratio of 6:1. In the Gd3+ containing system, 4 equivalents of NaOH did not have a significant influence, as Gd2(H2L) crystallized with and without NaOH in nearly

the whole temperature range at all excess metal-to-ligand ratios (Figure 4.3 - 1c). Exceptions in the presence of NaOH were the metal-to-ligand ratios of 2:1 and 6:1 at 75 °C, and the metal-to-ligand ratio of 2:1 at 180°C. Without NaOH,

Gd2(H2L) was not formed at the metal-to-ligand ratio of 4:1 at 100 °C and at the

metal to ligand ratio of 2:1 at 180 °C. In the Dy3+ containing system, the yellow crystallization field is small compared to the other lanthanides (Figure 4.3 - 1d). In the presence of four equivalents NaOH, pure phase Dy2(H2L) was observed at

75°C at metal-to-ligand ratios between 4:1 and 6:1. At 100°C it was formed at a metal-to-ligand ratio of 2:1. Without NaOH, Dy (H L) was generated at 100°C

H8L - A New Building Block in Metal Organic Framework Synthesis 4

and a metal-to-ligand ratio of 6:1. Furthermore, three unkown phases were observed.

La2(H2L), Nd2(H2L), Gd2(H2L) and Dy2(H2L) are isomorphous (Figure 4.3 - 8).

Raman spectra show that the ligand H2L was incorporated in the structure

(Figure 8.1.3 - 1). EDX analysis reveals an increased lanthanide to ligand ratio of 2:1 (Ln : P = 1 : 2) for all four compounds. Depending on the reaction media, they crystallize in the shape of large blocks or as intergrown platelets. Single crystalline blocks of La2(H2L) were obtained from concentrated and metal-rich

synthesis mixtures (Figure 4.3 - 9a). Under more dilute conditions, La2(H2L)

forms agglomerates of twisted sheets (Figure 4.3 - 9b). The structure could be solved from the single crystalline blocks. The simulated theoretical pattern fits well with the experimental data (Figure 8.1.1-2). Therefore the structure of

La2(H2L) can be discussed exemplarily for structure type II (Figure 4.3 - 1):

Single crystal structure analysis revealed the composition of La2[(HO3P)(O3P)-

CH-C6H4-CH-(PO3)(PO3H)]·8H2O. The asymmetric unit contains 17 non-

hydrogen atoms and 12 hydrogen atoms. The crystal structure is composed of [(HO3P)(O3P)-C6H4-(PO3)(PO3H)]6- ions, La3+-ions and water molecules (Figure

4.3 - 10a). According to the P-O distances, the phosphonic acid group around P1 is fully deprotonated and the phosphonic acid group around P2 is only once deprotonated (Table 8.1.5 - 8). In good agreement, one hydrogen atom could be located next to O5. The La3+-ion has eightfold coordination geometry (Figure 4.3 - 10b). It is coordinated by two bidentate chelating bisphosphonate units (O2; O1 and O4; O6), one bidentate coordinating phosphonate group (O3; O4) and two water molecules (O7, O6). The phosphonate La-O distances are in the range between 2.66 Å and 2.44 Å, which is comparable to those reported for LaPO4 166 (Table 8.1.5 - 9). The La-Owater distances are 2.51 Å and 2.61 Å. O4

coordinates to two La3+ ions. This associates two La3+ complexes into dimeric units (Figure 4.3 - 10c). The lanthanum dimers can be represented as two edge-

4 H8L - A New Building Block in Metal Organic Framework Synthesis

Figure 4.3 - 8: Powder pattern of a) Dy2(H2L), b) Gd2(H2L), c) Nd2(H2L) and d)

La2(H2L)

Figure 4.3 - 9: crystal morphologies of La2(H2L): a) single crystalline blocks b)

H8L - A New Building Block in Metal Organic Framework Synthesis 4

Figure 4.3 - 10: Structure of La2[(HO3P)(O3P)-CH-C6H4-CH-(PO3)(PO3H)] ·8H2O,

La2(H2L) (hydrogen atoms are omitted for clarity) : a) asymmetric unit with

completed molecular fragments b) coordination of La3+ c) association of two La3+ ions into a dinuclear cluster via two La-O-La bridges. d) 2D coordination network; e) projection along [100] showing a cross section of the layers.

sharing coordination polyhedra; they are connected along (100) into a chain by bridging phosphonate groups (Figure 4.3 - 10d). The chains are linked by the organic spacer of H2L, resulting in a two-dimensional coordination network in

the (001) plane. The remaining two crystallographically independent water molecules around O9 and O10 are located between the layers (Figure 4.3 - 10e). The layers stick together by weak interactions such as hydrogen bonds and Van- der-Waals interactions. The interlayer distance is 9.91 Å. In good agreement with this structure description, thermogravimetric analysis of the compounds

La2(H2L), Nd2(H2L), Gd2(H2L) and Dy2(H2L) show a weight loss of 16 – 18 % in

two endothermic steps between 60°C and 150°C as well as 150°C and 250°C that correspond to the loss of the eight stoichiometric equivalents of water in

4 H8L - A New Building Block in Metal Organic Framework Synthesis

the structure formula (Figure 8.1.4 - 2). The two steps can be explained by the different bonding modes of water. Coordinating water is supposed to leave the structure at higher temperatures compared to free crystal water.